It is recognized that hypercholesterolemia in man plays a significant role in predisposing individuals to premature clinical coronary artery disease. This correlation has stimulated much research into determining the genetic and environmental causes of hypercholesterolemia. Our studies of cells cultured from normal individuals and patients with abnormalities of cholesterol metabolism offer a direct method for examining the regulatory events involved in these human disease processes at the cellular level. The laboratory and clinical facilities that we have established, at Children's Hospital, Boston, to study inborn errors of lipid metabolism, provides access to a wide variety of children with these disorders from whom cell cultures may be established for study. In recent studies, we have discovered a new biochemical condition mimicing homozygous familial hypercholesterolemia, which may provide new insight into the regulation of cholesterol metabolism in man. In addition, our studies of the human fibroblast enzyme acid lipase will be continued utilizing the new sensitive assay system that we have developed. The fibroblast system will be used to study the regulation of acid lipase activity and to investigate the possible role of this enzyme in atherogenesis. We also propose studies on primary rat hepatocyte cultures to examine in detail the regulation of cholesterol biosynthesis by a cell type which governs a major portion of the synthetic and regulatory events of cholesterol metabolism in the intact animal. Techniques have only recently been available for the establishment of homogeneous non-proliferating monolayer cultures of adult rat hepatic parenchymal cells. This system is viable for a long enough period in vitro to study the mechanisms involved in the direct effects on liver cells of normocholesterolemic, hypercholesterolemic, and intestinal lymph lipoproteins. At the same time as rat liver cultures are being examined, rat fibroblasts will be compared in parallel studies to determine if regulation is similar in these two different cells from the same animal. Since rats are widely utilized for studies of cholesterol metabolism, it will be of interest to determine if the regulatory events seen in rat fibroblasts are similar to those in human fibroblasts. This combined approach of studies utilizing cultured normal and abnormal human cells as well as animal cells offers a useful method of obtaining information about the events which are intimately involved in the human arteriosclerotic disease process.